Tertiary rainbows

Rainbows of a higher order than the secondary bow are possible in theory. Three internal reflections of the rainbow ray would produce a tertiary bow, and four internal reflections would give a quaternary bow. In practice, these bows are much too faint to be seen against the brightness of the sky, through they can be observed in the laboratory. Figure 5.19 shows that the rainbow ray responsible for the tertiary bow emerges from a drop on the opposite side to those which cause the primary and secondary bows. This means that if you wanted to see a tertiary bow you would have to look at the sky in the vicinity of the Sun. Theory predicts that it should be a circular arc of diameter approximately 40° centred on the Sun with its red edge on the outside.

The possibility of a tertiary bow has been known ever since Edmund Halley calculated its position. Generations of experienced observers have taken up the challenge and have looked for it, though without apparent success. Ice halos and circumzenithal arcs are frequently mistaken for tertiary bows. If you want to take up the challenge of seeing a tertiary bow you should arrange for the Sun and the sky around it, right up to the point where this bow should appear, to be blocked off, say by a building. No one has ever reported seeing a quaternary bow in nature.

The most recent sighting of the elusive tertiary bow was made by D.E. Pegley, an experienced and reliable observer. As you can tell from his account of the event, although he was uncertain whether he had seen the bow, it is clear that he did not see an ice halo (section 7.1). So, what did he see?

Whilst in Nairobi recently I had the good fortune to see a tertiary rainbow. On 21 May 1986 at 17.55 a new shower cloud had just started to rain out over my hotel in dense curtains of medium-sized drops brilliantly lit by the low Sun. From the balcony of my fourth floor room I could see not only a bright primary, accompanied by a moderate secondary, but also a weak bow in the direction of the Sun, which was conveniently shielded by the side of the building. The bow was scintillating but distinct for two or three minutes. It was about the same size as the primary bow, with red on the outside and green on the inside . . . was it my imagination, or have other readers seen a tertiary rainbow . . .?

D.E. Pegley

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